The present invention relates generally to the field of telephony, and specifically to the detection of an off-hook condition while a ring signal is present.
In a typical line card for a telephony subscriber loop, the electrical characteristics of the loop are monitored. More specifically, the electrical load current (loop current) must to be monitored to perform supervisory functions and to support transmission of voice signals.
FIG. 1 illustrates a typical circuit 10 that is used for monitoring the loop current. During ringing, it is necessary that telephone systems be capable of detecting when the ringing telephone is taken off-hook, i.e., when the line is answered, and this detection must take place as rapidly and as reliably as possible.
It is desirable to detect the status of a looped subscriber line regardless of whether the subscriber is making a call or receiving a call. When the subscriber is initiating a call, loop detection is not difficult. However the same is not true for a subscriber receiving a call, i.e., when a subscriber unhooks a ringing telephone. In current telephone systems, a direct voltage is superimposed on the ringing waveform, and the loop is detected by virtue of a direct current (DC) component which flows when the telephone is taken off-hook. Such detection becomes very difficult when the alternating current (AC) in the line due to the AC component of the ringing current is considerably greater than the DC component to be detected. Further, the wide variety of line impedances and ringing impedances makes it necessary to design for worst case telephone systems which are generally not optimum in performance for the majority of telephone installations.
As illustrated, a subscriber is connected to a telephone exchange by the tip and ring lines each having a value of line resistance RLoop/2, and a ringer is connected to the terminals of the line. The ringer can be approximated by a resistance Zset connected in series with a capacitance Cs. Likewise across the terminals of the line, and thus in parallel with the ringer, there is the remainder of the subscriber station which is equivalent to a resistance Rt connected in series with a switch S1, where the switch represents the hook-switch of the telephone set. The line is supplied with power by a DC source which delivers a direct voltage of typically 48 volts in most cases. A ringing source formed by an AC ring generator supplies the line with an r.m.s. voltage of 80 VAC.
In the on-hook mode, i.e., for a called subscriber, only the AC generator causes a current to flow, and the value of the current is determined by the line resistance and by the ringer impedance, i.e., the resistance Zset and the capacitance Cs in series. In an offhook hook mode, i.e., once the subscriber has taken the local telephone off hook, the DC generator delivers direct current at a value which is determined by the line resistances and Rt, and the AC generator delivers a current which is determined by the resistance of the line in series with an impedance defined by the resistance Rt in parallel with the impedance of the ringer, and thus in parallel on the series connection of the resistance Zset and the capacitance Cs. The line current in off-hook mode thus comprises a DC component and an AC component.
Known techniques use the loop current for determining an off-hook condition. In a Plain Old Telephone System (POTS), the determination that the subscriber has gone off hook while the ring signal is still present on the loop requires a combination of discrete components and integrated operational amplifiers (op amps), or complex analog integrated filters to replace the discrete components.
FIG. 2 illustrates several known circuits that are used for determining the off-hook condition. When a ringing signal is applied and the subscriber is on hook, a switch S1 is open. Therefore, AC ringing loop current flows through the circuit, but DC loop current does not. When the subscriber goes off hook, the switch S1 is closed, and the loop current is the sum of the AC ringing loop current and the DC loop current. In order to determine if sufficient DC loop current on average is flowing for indicating that the subscriber is off-hook, the AC loop current present in the sensed loop current is filtered. The remaining current is then compared to a threshold value.
The AC ringing loop current is filtered from the sensed loop current with either external discrete components, usually single-pole 12 or double-pole 14 passive circuits, or integrated amplifiers 16 with on-chip or off-chip filter components, or a more complex integrated ring trip filter 18. The filtered current is then passed through a comparator 20 for comparing it to a threshold value 22.
These solutions, however, are costly in terms of required components, area, and power. Component selection is a trade-off between the speed of detection and debouncing of detection, for achieving a minimal delay period for threshold detection.
What is needed is a mechanism to obviate or mitigate at least some of the above disadvantages.
In accordance with the present invention, a line card is provided for terminating a telephone line and for coupling to customer premises equipment (a telephone) wherein the line card has an interface for transmitting and receiving an analog signal along the telephone line, an AC ring signal generator for generating a ring signal, a DC signal generator for generating a DC signal when said customer equipment is off-hook, and a circuit for determining the duty cycle of a loop current in the telephone for determining the hook status of the customer equipment.
In accordance with an embodiment of the invention, a method is provided for determining a hook status of a customer""s telephone equipment comprising the steps of detecting a loop current having a duty cycle dependant on customer equipment hook status, determining the duty cycle of the loop current, and comparing the duty cycle to a threshold, the comparison being used for determining the customer hook status.